Ceramic whiskers comprising silicon nitride, silicon carbide, alumina, aluminum borate, potassium titanate, titanium oxide, etc. are excellent in mechanical characteristics, heat resistance, corrosion resistance, heat insulating properties, wearability, and electrical insulating properties and have been their use has been extended into many fields such as WRM, WRP (whisker reinforced plastic), and WRC (whisker reinforced ceramic).
On the other hand, with recent developments in of the automobile industries and the aircraft and aerospace industry, there has been an increasing demand for high-strength and high-modulus metallic materials which are light and excellent in heat resistance and wearability. Due to the increase in cases in which conventional aluminum alloys cannot meet the demand, reinforcement of metal with heat-resistant fibrous materials or whiskers has been attempted, and many proposals have been made on metallic materials reinforced by this method, i.e., metal-matrix composite materials.
Whiskers comprising silicon nitride, silicon carbide, alumina, aluminum borate, potassium titanate, titanium oxide, etc. can be exemplified as the typical reinforcements which have hitherto been used for production of metal-matrix composite materials. Since it is difficult to disperse extremely fine whiskers in a molten metal uniformly, whisker-reinforced metal-matrix composite materials have generally been obtained by previously forming whiskers into a porous body called a preform and infiltrating a molten metal into the pores of the preform.
A whisker preform has conventionally been prepared by dispersing whiskers and a binder in water and filtering the aqueous dispersion with suction, followed by drying and calcination. The conventional process has several problems. Firstly, whiskers, which are light and fine, are liable to scatter when dispersed in water and, if inhaled, may endanger workers' health. A strict countermeasure should be taken against scattering. Secondly, while a whisker dispersion is filtered, the whiskers having a needle shape are unavoidably oriented in a specific direction or localized (unevenly distributed), and the orientation and localization remain in the resulting preform. Needless to say, a uniform reinforcing effect cannot be expected from a preform having whiskers oriented or unevenly distributed. Thirdly, since the physical properties of a preform, such as strength and heat resistance, are considerably influenced by the binder used in combination, the excellent properties essential to whiskers cannot be taken full advantage of. Fourthly, it is difficult to obtain a preform having the density and dimensions as originally designed because of considerable change in volume occurring during filtering up to the completion of firing. Another problem is that the whisker preform is extremely expensive because, for one thing, the whiskers as a raw material are expensive.
The above-mentioned problems have been pointed out with respect to not only a whisker preform itself but various products utilizing a whisker preform, and have made it difficult to take full advantage of the excellent properties of whiskers. Furthermore, when the conventional whisker preform is used for reinforcement of metal, it has the following problems. Since the bonding strength among whiskers in a preform is not so strong, the preform tends to be deformed or broken when a molten metal is infiltrated therein. The binder in a preform tends to react with a molten metal, e.g., aluminum, to cause reduction in strength.
JP-B-6-33193 (the term "JP-B" as used herein means an "examined published Japanese patent publication") discloses a process for producing rigid mullite whisker felt, the process having succeeded in solving the problems associated with preparation of whisker preforms as far as mullite whisker is concerned. The disclosed process comprises heating a green compact comprising a mixture of AlF.sub.3 and SiO.sub.2 in an anhydrous SiF.sub.4 atmosphere at about 700 to 950.degree. C. to form rod-like topaz crystals from AlF.sub.3 and SiO.sub.2, and further heating the topaz crystals at about 1150 to 1700.degree. C. to convert the topaz crystals to needle-like single crystal mullite whiskers. According to this process, since mullite whiskers are grown by the reaction in a compact, the mullite whiskers are bound to each other during the growth, and a whisker felt is completed on the completion of whisker growth. Therefore, the whisker filtering process for forming a whisker preform, which is accompanied with many difficulties, is unnecessary, and the resulting product is therefore free from the above-described drawbacks arising from filtering.
However, this process, while seeming advantageous, is not easy to carry out because of the involvement of additional cost for the equipment and the process control due to the calcination in a toxic anhydrous SiF.sub.4 atmosphere. Besides, the product is limited to mullite materials, and the applicability of the technique is limited. A process for producing whiskers other than mullite whiskers in which whisker formation and formation of a porous body are carried out at the same time is unknown.